Series connected flip chip LEDs with growth substrate removed

ABSTRACT

LED layers are grown over a sapphire substrate. Individual flip chip LEDs are formed by trenching or masked ion implantation. Modules containing a plurality of LEDs are diced and mounted on a submount wafer. A submount metal pattern or a metal pattern formed on the LEDs connects the LEDs in a module in series. The growth substrate is then removed, such as by laser lift-off. A semi-insulating layer is formed, prior to or after mounting, that mechanically connects the LEDs together. The semi-insulating layer may be formed by ion implantation of a layer between the substrate and the LED layers. PEC etching of the semi-insulating layer, exposed after substrate removal, may be performed by biasing the semi-insulating layer. The submount is then diced to create LED modules containing series-connected LEDs.

This application is a Divisional of U.S. patent application Ser. No.12/266,162, filed 6 Nov. 2008 and is incorporated by reference herein.

FIELD OF THE INVENTION

This invention relates to forming light emitting diodes (LEDs) and, inparticular, to a method for simultaneously forming interconnected LEDs.

BACKGROUND

Individual LEDs have been connected in series across a mains voltage(e.g., 120 VAC) so that the combined forward voltages of the LEDs areslightly less than the peak voltage. Therefore, the LEDs briefly turn oneach AC cycle. The rapid blinking of the LEDs is not perceived by thehuman eye. Such a light source is inexpensive since no DC currentregulator is needed. However, since many LEDs are used (typically30-60), mounting and interconnecting individual LEDs on a circuit boardcreates a relatively large light emitter.

Serially-connected LEDs are also useful for DC applications, such asautomobiles, where parallel strings of three LEDs are connected acrossthe battery voltage (nominally 12 volts) to create any brightness level.

A series resistor is typically used to limit the current in such LEDstrings not using an active current source. The LEDs may be any colorand may even have a phosphor coating to generate white light.

Recently, the efficiency of GaN LEDs has been increased by the removalof the transparent sapphire growth substrate, used to grow GaN LEDlayers. After the removal of the substrate, the exposed GaN layer isetched to thin the layer and to create a roughened surface to increaselight extraction.

What is needed is an efficient technique to form series LEDs in acompact structure. What is also needed is such a compact LED structurethat can reliably undergo the substrate removal process. What is alsoneeded is such a compact LED structure that can also reliably undergothe etching process, after substrate removal, to increase lightextraction.

SUMMARY

A wafer-scale process is described that forms any number of seriallyconnected LEDs on a single submount. In one embodiment, a string of blueLEDs with an overlying phosphor layer is fabricated to generate anycolor.

In one embodiment, a sapphire growth substrate wafer has epitaxiallygrown over it a semi-insulating (SI) GaN layer that is relatively thick.The resistivity of the SI-GaN layer is controlled by doping to have aresistivity on the order of 10⁵-10⁶ ohm-cm. N-GaN layers are grown overthe SI-GaN layer, followed by an active layer and P-GaN layers. Trenchesare etched through the LED layers until the trench reaches the SI-GaNlayer. The trenches create an X-Y grid to form an array of isolatedLEDs. All the GaN layers are transparent.

The LEDs may be interconnected by a metal layer deposited over the LEDsto form the serial connection between any number of LEDs. Alternatively,the serial connection may be achieved by a metal pattern on a submount.The LEDs may also be connected in both series and parallel, or inanother arrangement.

In one embodiment, groups of LEDs that are intended to be in a singlemodule of serially-connected LEDs are separated out, such as by sawingthe sapphire substrate to separate out the modules. Each LED module,supported by a portion of the sapphire substrate and the SI-GaN layer,is then mounted on the submount wafer. The LEDs are formed as flip-chipsso that the sapphire substrate is face up, opposite to the submountsurface. Alternatively, prior to dicing, the wafer is aligned with andmetal-bonded to the submount wafer.

The sapphire substrate is then removed from each module on the submountwafer by a laser lift-off process. The laser causes the top layer of GaNto release a gas to push off the sapphire substrate from the GaN layer.The laser lift-off process generates a large downward pressure on theLED layers. The thick SI-GaN layer mechanically supports the thin N-GaN,active, and P-GaN layers during the lift-off process to preventbreakage. Once the substrate has been removed, the SI-GaN layercontinues to provide mechanical support to the individual LEDs since itmechanically connects them together yet does not adversely electricallyconnect them.

To thin the LEDs and improve the light extraction through the top GaNlayers, a photo-electrochemical (PEC) etch of the exposed surface isperformed. A PEC etch involves electrically biasing the material to beetched, immersing the material in a base solution, and exposing thematerial to UV light. The submount has a grounded metal trace patternthat contacts the N-GaN layer of the LEDs, where the metal pattern isjust used to electrically bias the SI-GaN layer through the N-GaN layerduring the PEC etch process. The SI-GaN layer has a resistance that issufficiently high so as to not short out the series LEDs butsufficiently low to allow the SI-GaN layer to be electrically biasedduring the PEC etch. Without doping the GaN layer to create the SI-GaNlayer, the resistance would be too high to efficiently conduct the PECetch. After the PEC etch, the metal traces on the submount used for thebiasing are cut or blown. A PEC etch is optional.

In another embodiment, instead of growing a SI-GaN layer, a P-type orN-type or undoped GaN layer is grown on the sapphire substrate, followedby a blanket ion implant. The ion implant creates defects in thecrystalline structure of the GaN layer, making it semi-insulating. Thenormal LED layers are then grown over the SI-GaN layer. The LED layersare then trenched down to the SI-GaN layer to electrically isolate theindividual LEDs. The LEDs/substrate are then diced to create the LEDmodules with series-connected LEDs, and the LEDs are then mounted asflip chips onto a submount wafer. The substrate is then removed fromeach of the modules on the submount wafer by laser lift-off. The LEDsfor a module are connected in series by a metal pattern. The SI-GaNlayer helps provides mechanical support to the LEDs during substratelift off and after the substrate is removed. An optional PEC etch of theexposed surface may then be performed by biasing the SI-GaN layer duringthe PEC etch.

In another embodiment, LED layers are grown on the substrate. Trenchesare then formed through the P-layers and active layer and only part waythrough the N-layers. After dicing to separate the LED modules, the LEDsare then mounted as flip chips onto a submount wafer, and the substrateis removed by laser lift-off. The LEDs for a module are connected inseries by a metal pattern. The exposed surface is then ion implanteddown to where the trenches ended so that the top portion of the N-layersis semi-insulating. The semi-insulating portion mechanically connectsthe LEDs together for support. An optional PEC etch of the exposedsurface may then be performed by biasing the surface layer during thePEC etch.

In another embodiment, LED layers are grown on the substrate, and thelayers are masked to expose the boundaries between LEDs. Ions are thenimplanted into the boundary regions of the LEDs through the P-layers,active layer, and N-layers to render those portions of the LED layerssemi-insulating. No trenching is needed for electrically isolating theLEDs. Since there is no trenching, the LEDs are mechanically connectedtogether by the ion implanted areas after the growth substrate isremoved. The LEDs are connected together in a serial configuration by ametal pattern. The exposed surface of the LEDs, after mounting on thesubmount, may optionally be subjected to the PEC etch while biasing theexposed layer. The ion implanted areas occupy a small percentage of thesurface area at the edges of the active device and need not be roughenedto achieve good light extraction.

After the optional PEC etch, a phosphor layer may then be deposited overthe LEDs, where the phosphor layer combined with blue light leakingthrough creates any color of light.

The submount wafer is then diced to separate the LED modules or unitsthat contain any number of serially connected LEDs. An AC voltage or ahigh DC voltage may be connected to submount electrodes to energize astring of LEDs.

Aspects of the process may be applied to LEDs that are not GaN, such asAlInGaP LEDs. Any suitable substrate, such as a GaN substrate, may beused to grow the various layers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified cross-sectional view of a blue LED formed inaccordance with one embodiment of the invention.

FIG. 2 is a simplified perspective view of a small portion of a wafershowing three LEDs formed on it and isolated using trenches.

FIG. 3 illustrates the wafer-level metallization to form serialconnections between the LEDs.

FIG. 4 is a schematic view representing any number of LEDs connected inseries.

FIG. 5 illustrates an alternative to FIG. 3, where co-planer bondingpads on the LEDs are formed for connection to a metal pattern on asubmount, where the metal pattern on the submount creates the serialconnection between the LEDs.

FIG. 6 illustrates a laser lift-off process to remove the sapphiresubstrate after the LEDs have been mounted on a submount wafer.

FIG. 7 is a top down view of one example of the layout of six LEDs onthe submount, showing the areas of the P-metal and N-metal ohmiccontacts on the LEDs bonded to a metal pattern on the submount. Only thegrounded metal pattern for temporarily connecting the SI-GaN layer to abias voltage during a PEC etch is shown for simplicity.

FIG. 8 is a side view showing the top layer of the LEDs, mounted on thesubmount wafer, undergoing a PEC etch to improve light extraction, whilethe SI-GaN layer is electrically biased.

FIG. 9 is the same view as in FIG. 7 but showing a schematicrepresentation of the serial connection between the LEDs provided by ametal pattern on the submount. The biasing traces shown in FIG. 7 havebeen cut or blown.

FIG. 10 is a flowchart showing various steps used in one embodiment ofthe invention, where trenches, in conjunction with a semi-insulatinglayer, is used to isolate the LEDs.

FIG. 11A illustrates another embodiment where ions are implanted in aGaN layer to create a semi-insulating layer.

FIG. 11B illustrates the structure of FIG. 11A after LED layers areformed over the semi-insulating layer and after trenches are formed toisolate individual LEDs.

FIG. 11C illustrates the structure of FIG. 11B after being mounted to asubmount and after the substrate is removed.

FIG. 12A illustrates another embodiment where LED layers are etched toform trenches that partially extend into the N-layers.

FIG. 12B illustrates the structure of FIG. 12A after being mounted on asubmount, after the substrate is removed, and after an ion implant tomake the top surface portion semi-insulating.

FIG. 13 illustrates another embodiment where a masked ion implantationthrough the LED layers down to the substrate isolates the individualLEDs.

FIG. 14 is a flowchart showing various steps used in one embodiment ofthe invention where trenching and/or ion implantation is used toelectrically isolate the LEDs.

Elements identified by the same numeral in the various figures are thesame or similar.

DETAILED DESCRIPTION

FIG. 1 is a simplified cross-sectional view of an LED 10 formed inaccordance with one embodiment of the invention. In the examples, blueLEDs are formed on a wafer. However, other types of LEDs may be formedinstead, such as AlInGaP LEDs. All the layers are epitaxially grown.

On a sapphire substrate 12 is grown a conventional GaN nucleation layer,a growth initiation layer (GIL), and a coalescence layer, allrepresented by the layer 14. Such layers are generally used to provide atransition between the sapphire substrate 12 lattice constant and theLED layers' GaN lattice constant to minimize the defect density in thecrystalline structure of the critical N-GaN, active, and P-GaN layers.Forming such transition layers are described in U.S. Pat. Nos. 6,989,555and 6,630,692, assigned to the present assignee and incorporated byreference.

Over layer 14 is grown a thick semi-insulating (SI) GaN layer 16. TheSI-GaN layer 16 performs at least two functions. It provides mechanicalsupport for the LED layers during a substrate laser lift-off process,and it is electrically biased for a photo-electrochemical (PEC) etchprocess while not being sufficiently conductive to short out the LEDs.In one embodiment, the layer 16 is 2 microns thick or greater. Theresistance of the SI-GaN layer 16 may be controlled by doping using Feor Mg. A SI-GaN layer is described in the paper, “Electrical and OpticalProperties of Fe-Doped Semi-Insulating GaN Templates,” by A. Y.Polyakov, Appl. Phys. Lett. 83, 3314 (2003), incorporated herein byreference.

The resistivity of the layer 16 should be chosen to provide adequateelectrical isolation between LEDs (after a trench is formed between theLEDs) yet allow a current of about 10 uA per mm² to flow with a voltageof approximately 0.1 volt. Such characteristics are needed during a PECetch, described later. A resistivity of 100 Kohm-cm would be adequate.An acceptable range of resistivity may be 10⁴-10⁶ ohm-cm. The voltagebetween neighboring LEDs (after a trench is formed) would be about 4volts and, therefore, a leakage current across a 10 um wide trench wouldbe about 8 uA per element.

If instead of a string of LEDs, a two dimensional array of LEDs areconnected in series, then the leakage current would be higher as theaverage voltage difference between devices and the number of neighboringdevices would be greater. However for practical embodiments, a SI-GaNresistivity on the order of about 10⁵ ohm-cm would only permit a sub-mAleakage current

Over the layer 16 are formed conventional N-GaN confining layers 18, aconventional active layer 20, and conventional P-GaN confining layers22. In one embodiment, the active layer 20 is AlInGaN and generates bluelight.

Since the LEDs are to be flip-chips, with the N and P contacts on thesurface of the LEDs facing the submount, the P-layers are masked andetched to expose portions of the underlying N-layer. Then, as shown inFIG. 2, trenches 24 are etched between LED portions using reactive ionetching. Etching trenches to isolate GaN LEDs is described in U.S. Pat.No. 6,547,249, assigned to the present assignee and incorporated hereinby reference. The trenches 24 are etched down to the SI-GaN layer 16.

Metal ohmic contacts 26 to the P and N semiconductor layers are formedbefore or after the trenches 24 are formed.

Although only three isolated LEDs are shown in FIG. 2 for simplicity,there will normally be many hundreds of isolated LEDs simultaneouslycreated on the same growth substrate.

FIG. 3 illustrates a metallization process for interconnecting the LEDsin one embodiment. A planarized insulating layer 30 may be formed overthe surface to fill the trenches 24 and create a substantially flatsurface. Si₂N₃ or other dielectric may be deposited. The insulatinglayer 30 is masked and etched to expose the ohmic contacts 26 on the Nand P layers. A metal is then deposited, such as an aluminum alloy, tocreate serial interconnections 32 between the LEDs. The interconnections32 are shown schematically for simplicity. In an actual embodiment,wider metal strips would make the interconnections.

FIG. 4 is a schematic of a possible serial connection between theindividual LEDs 36 that may be made by the interconnections 32. Anynumber of LEDs 36 may be connected in series and in parallel to achievethe required forward voltage drop and brightness. An AC source 38 isshown coupled to the serial strings. Since the AC voltage is assumed togo positive and negative during each cycle, the LED strings areconnected in an anti-parallel configuration, so that the stringsalternately turn on. The LEDs 36 in a string will turn on when the ACvoltage rises above the forward voltage. A current limiting resistor 40sets the maximum current through each string. If the AC source is themains voltage (e.g., 120 volts, 220 volts, etc.), there may 30 or moreLEDs 36 in a string. In another embodiment, a full wave bridge rectifieris formed using silicon diodes to create a rectified signal, and aseries string of LEDs is driven by the rectified signal. Such anembodiment uses half the number of LEDs as an anti-parallelconfiguration for the same brightness output. The rectifier may beformed on the submount.

As an alternative to depositing the metal interconnections on the LEDs,the interconnections may be a metal pattern on a submount. FIG. 5illustrates raising all the N-metal contacts 42 to be coplanar with theP-metal contacts 26 so the interconnections can be made by the submounttrace pattern. An insulating layer, similar to that shown in FIG. 3, mayfirst be formed, followed by depositing the raised metal contacts 42into openings in the insulation layer. The submount interconnectionpattern may resemble the pattern of the interconnections 32 in FIG. 3.

The LEDs/substrate is then diced, such as by sawing or scribe-and-break,to form modules of LEDs, containing any number of LEDs. Each module maybe rectangular to simplify the sawing process. Each module would laterbe packaged in a single package after all other processing.

In FIG. 6, the separated modules from the structure of FIG. 3 or 5 arethen mounted on a submount wafer 44. Only one simplified module in shownin FIG. 6 on a portion of the submount wafer 44.

A metal pattern on the submount wafer 44 leads from the LED metalcontacts 26/42 to more robust metal pads for connection to a circuitboard. Pads on the submount wafer 44 are bonded to corresponding pads onthe LEDs by either ultrasonic welding, soldering, or other method. Goldbumps may act as an interface metal. Bonding LEDs to a submount isconventional.

In FIG. 6, the N-layers, the active layer, and the P-layers are shown aslayer 46. The sapphire substrate 12 is removed by laser lift-off. Thelaser energy is shown by arrows 48. The photon energy of the laser(e.g., an excimer laser) is selected to be above the band gap of the LEDmaterial and below the absorption edge of the sapphire substrate (e.g.,between 3.44 eV and 6 eV). A pulse from the laser through the sapphireis converted to thermal energy within the first 100 nm of the LEDmaterial. The generated temperature is in excess of 1000° C. anddissociates the gallium and nitrogen. The resulting high gas pressurepushes the substrate away from the epitaxial layers to release thesubstrate from the layers, and the loose substrate is then simplyremoved from the LED structure. The presence of the SI-GaN layer 16blocks the laser radiation from reaching the submount surface in theregion between the LEDs so as to prevent potential damage to thesubmount.

The mechanical support provided by the thick SI-GaN layer 16 preventsthe tremendous downward pressure on the LED layers 46 from cracking theLED layers 46. The SI-GaN layer 16 also provides mechanical support ofthe individual LEDs after the growth substrate 12 is removed.

The growth substrate 12 may instead be removed by etching, such asreactive ion etching (RIE), CMP, or grinding. Suitable substrate removaltechniques are described in U.S. Pat. No. 7,256,483, entitled,Package-Integrated Thin Film LED, by John Epler et al., incorporatedherein by reference.

After the growth substrate is removed, the exposed surface of the layer14 or layer 16 is not optimal for light extraction through the surface.For thinning the LED and to achieve an optimal roughened surface of thelayer 16, photo-electrochemical (PEC) etching is used. Removal of thelayer 14 prior to PEC is optional since the electrical field createdbetween the SI-GaN layer 16 and the base solution (described below)would still exist with the layer 14 in-between. The layer 14 may beremoved by PEC etching or non-PEC etching, such as by RIE, CMP, orgrinding, to reveal the SI-GaN layer 16.

In PEC etching, an electric field should be created between the surfaceto be etched and the solution to increase the rate of etching andcontrol the etch rate. To enable such biasing, a sacrificial thin metalpattern is provided on the submount for grounding the SI-GaN layer 16via the N-layer 18 metal contacts.

FIG. 7 is a top down view of a portion of the submount wafer 44supporting a single LED module of eight LEDs, showing the P and Ncontacts 26/42 on the LEDs, the SI-GaN layer 16 connecting the LEDs, andthe sacrificial metal pattern 50 on the submount wafer 44. The patternof the P and N contacts in FIG. 7 shows the N contact 42 surrounding theP contact 26 for symmetrical current flow through the active layer. Inprevious embodiments, the representation of the N and P contacts wassimplified. The SI-GaN layer 16 is connected to ground, via the metalpattern 50 and N-layer, at many distributed points, otherwise the highresistance would prevent uniform biasing of the layer 16, and an unevenPEC etching would occur.

In FIG. 8, the PEC etch 51 of the exposed surface of layer 14 or 16 isperformed. At least layer 14 is etched through during the PEC etch. Toperform the PEC etching, at least the layer to be etched is immersed ina base solution, and an electrode with a positive potential is immersedin the base solution. An example of a suitable base solution is 0.2 MKOH, though many other suitable basic or acidic solutions may be usedand depend on the composition of the material to be etched and thedesired surface texture. The epitaxial surface of the GaN layer 14/16 isexposed to light with energy greater than the band gap of the surfacelayer. In one example, ultraviolet light with a wavelength of about 365nm and an intensity between about 10 and about 100 mW/cm² is used.Exposure to the light generates electron-hole pairs in the surfacesemiconductor layer. The holes migrate to the surface of the GaN layerunder the influence of the electric field. The holes then react with theGaN at the surface and base solution to break the GaN bonds, accordingto the equation 2 GaN+6OH⁻+6e⁺=2Ga(OH)₃+N₂. The current through theSI-GaN layer 16 is about 10 uA for a 1×1 mm² LED. The PEC voltage shouldbe kept below the diode breakdown voltage (e.g., below 5 volts).Additional detail of PEC etching of a GaN layer may be found in USpublication 20060014310, by John Epler, assigned to the present assigneeand incorporated herein by reference.

The resulting roughening of the surface of layer 16 reduces the internalreflections within the LED structure to increase efficiency.

After the PEC etch, a laser may be used to cut the metal pattern 50(FIG. 7) so that the N-contacts are no longer shorted together andconnected to ground. Alternatively, a sufficiently high pulse of currentmay be passed through the pattern 50 to burn out the thin metal.

FIG. 9 is the same view as in FIG. 7 but shows the interconnects 52between the LEDs formed by either a metal pattern deposited on the LEDs(FIG. 3) or a metal pattern formed on the submount wafer 44 forcontacting the P and N metal contacts 26/42 on the LEDs (FIG. 5). It ispreferred to interconnect the LEDs so that there are no large voltagedifferences between adjacent LEDs. Leads 54 on the submount wafer 44extend beyond the LEDs for connection to a power supply, such as an ACor DC supply. The leads 54 may be on the same surface as the LEDs or onthe bottom surface of the submount wafer 44 for direct connection topads on a circuit board.

Wafer level phosphor deposition and encapsulation may be performed bymolding, deposition, or other technique. If the LEDs emit blue light,the phosphor layer can contain green and red phosphors so that thecombination of the leaked blue light with the green and red lightcreates white light. A yellow-green YAG phosphor may be used instead. Alens may be formed over each LED module by molding.

The submount wafer 44 is then diced, such as by sawing or scribe andbreak, to create modules containing LEDs connected in series, where theLEDs in a module are mechanically coupled together by the SI-GaN layer16. The modules may then be packaged.

FIG. 10 is a flowchart identifying various steps used in one embodimentof the invention.

In step 64, a thick semi-insulating layer is epitaxially grown over asapphire substrate. The resistance of the semi-insulating layer iscarefully controlled to conduct current but not short out thelater-formed LEDs.

In step 65, the LED layers are epitaxially grown over thesemi-insulating layer.

In step 66, LEDs are electrically isolated from each other by trenchesthrough the LED layers but not extending through the semi-insulatinglayer. The thick semi-insulating layer helps keeps the individual LEDsaligned after the sapphire substrate is later removed, especially if thesubmount is flexible. Further, if the submount undergoes hightemperature processing, the semi-insulating layer resists misalignmentof the LEDs resulting from any metal creep. Still further, since thesemi-insulating layer covers the edges of each individual LED, and edgesof a die are the most vulnerable to humidity, the semi-insulating layerreduces this vulnerability. Each LED is a flip-chip (both P and Ncontacts on surface facing away from light extraction surface).Optionally, the sapphire substrate is sawed to separate out LED modules,each module containing LEDs that are to be in a single package.

In step 67, after dicing the LED modules, the flip-chip LEDs are mountedon a submount wafer. The submount may electrically interconnect the LEDsin series, or in both series and parallel, or some other arrangement.The submount also has metal traces for biasing the commonsemi-insulating layer for a PEC etch. The submount wafer also providesmechanical support for the LEDs.

In step 68, the sapphire substrate is removed, such as by laserlift-off.

In step 69, a bias voltage (e.g., ground) is applied to thesemi-insulating layer via the N-contacts of the LED layers during a PECetch of the exposed layer. Leads on the submount wafer may be used forapplying the bias voltage to the semi-insulating layer.

In step 70, the leads on the submount wafer for biasing thesemi-insulating layer are cut or blown.

In step 71, the submount wafer is diced to form LED modules or units,each module containing a group of interconnected LEDs, such ascontaining one or more strings of LEDs for coupling to a relatively highvoltage supply.

FIG. 11A illustrates a GaN layer 80 grown over a sapphire substrate 82.The layer 80 thickness may be between 0.5-4 microns, and the thicknessis not critical. The layer may be P-type, N-type, or undoped. Ionimplantation is then performed to implant ions 83 into the layer 80. Theion implantation makes the layer 80 semi-insulating by creating defectsin the crystalline structure of the layer 80. The ion (proton)implantation is conducted with an approximate dose and energy of 8E13cm⁻² and 400 keV, respectively, for a 4 micron thick epi layer. Theoptimum dose and energy are determined experimentally and may vary by+/−50% from these values (e.g., depending on the layer thickness). Thedose and energy should be optimized to achieve sufficient isolationwhile minimizing implant induced optical absorption. Species include(but are not limited to) He, Zn, Al, and Mg. Ion implantation forcreating an insulating region in GaN material is well known in the art.

As shown in FIG. 11B, one or more thick N-layers 84 are then grown overlayer 80, followed by growing an active layer 86 and P-layers 88.

Trenches 90 are then formed through the LED layers 88, 86, and 84 downto the semi-insulating GaN layer 80. The etching is performed using aphotoresist mask followed by reactive ion etching 92 (RIE). The trenches90 isolate the LEDs, such as in FIGS. 2 and 7, so the LEDs can later beconnected together in series. The SI-GaN layer 80 has a very highresistivity so does not affect the series configuration. Any etchingneeded to form flip chips is also performed.

As shown in FIG. 11C, metal contacts 26 and 42 are formed to contact theP and N layers of the isolated LEDs. The LED modules are then diced, andthe LEDs are mounted on a submount 44. One module is shown in FIG. 11C.The LED layers are shown as layer 94 for simplicity. The substrate 82 islifted off using laser energy 48, as previously described. The LEDlayers may be supported for laser lift-off by making the metal contacts26 and 42 between the LED layers and the submount wafer 44 cover a largeportion of the bottom surface. The metal contacts may coverapproximately 90% of the area under the LED layers. The SI-GaN layer 80over the LED layers, after the substrate is removed, is electricallybiased by a metal pattern on the submount, as previously described, anda PEC etch is performed to optimize light extraction. The widths of thetrenches would be much smaller, relative to the size of the LEDs, thandepicted in FIG. 11C. The SI-GaN layer 80 mechanically supports the LEDswhile on the submount 44.

The remainder of the processing may be the same as previously describedto create separated LED modules containing LEDs in series.

In all embodiments, an underfill (e.g., silicone) may be injectedbetween the LEDs and the submount 44 to help support the LED layers andSI-GaN layer 80 during the laser lift-off and protect the LEDs.

FIGS. 12A and 12B illustrate another process using ion implantation forcreating a mechanically supporting layer between individual LEDs. InFIG. 12A, N-layers 84, the active layer 86, and P-layers 88 are grownover the substrate 82. Trenches 96 are formed through the P-layers 88and active layer 86 and partially through the N-layers 84 using a maskand RIE 98. The trenches 96 isolate the individual LEDs, as previouslymentioned.

In FIG. 12B, metal contacts 26 and 42 are formed to create flip chipsand, after dicing, the LED modules are mounted on the submount wafer 44.The substrate is then removed using laser lift-off. The LED layers areshown as layer 94 for simplicity. The top surface 100 is then implantedwith ions 102, using the same technique described with respect to FIG.11A, so that the top of the N-layer is made semi-insulating down towhere the trench 96 ends. The remainder of the processing to form seriesconnected LEDs in a module is the same as in previous embodiments.

FIG. 13 illustrates another embodiment where there are no trenchesformed to isolate the LEDs. The LED layers 84, 86, and 88 are grown overthe substrate 82. A photoresist mask 104 is then formed over thestructure to expose regions that will be semi-insulating boundariesbetween individual LEDs. Ions 106 are then implanted at a high enoughenergy to create defects in the crystalline structure in the layers 84,86, and 88 in the boundary areas to make those boundary areassemi-insulating to effectively electrically isolate the LEDs from eachother. The mask 104 is then removed. As in previous embodiments, metalcontacts are formed, the modules are diced, the modules are mounted on asubmount wafer, the LEDs are connected in series, the substrate isremoved, and the LED modules/submounts are separated out. The ionimplanted areas occupy a small fraction, approximately 8%, of theemitting area at the border between devices and need not becomeroughened by the PEC process to achieve good light emission.

Phosphor deposition and encapsulation may be performed on a wafer scale(before dicing) as previously described.

Various other metal connections between the LEDs may be used other thana simple serial connection. The LEDs may be connected to a controlcircuit that dynamically changes the number of LEDs connected in seriesacross an AC power supply based on the instantaneous voltage so that theLEDs are on for a longer time.

In all embodiments, the semi-insulating layers or regions should have aresistivity greater than about 10⁴ ohm-cm (greater than the resistivityof intrinsic GaN), but a lower resistivity may be an acceptable tradeoffto achieve less optical absorption due to the ion implantation.

FIG. 14 is a flowchart that broadly describes the processes that use ionimplantation, with or without trenches, to electrically isolate theLEDs.

In step 110, the LED layers are epitaxially grown over a substrate.

In step 112, LEDs are electrically isolated from each other by ionimplantation and, in some cases, trenches, as described with respect toFIGS. 11A through 13. In all cases, individual LEDs are mechanicallyconnected together by a semi-insulating layer formed by ionimplantation. The mechanical connection improves the robustness of theLEDs, helps align the LEDs, and provides additional support duringsubstrate lift-off. Each LED is a flip-chip (both P and N contacts onsurface facing away from light extraction surface).

In step 114, the modules are diced, and the flip-chip LEDs are mountedon a submount wafer. The submount may electrically interconnect the LEDsin series, or in both series and parallel, or some other arrangement.The submount also has metal traces for biasing the commonsemi-insulating layer (the top surface) for a PEC etch. The submountwafer also provides mechanical support for the LEDs. The sapphiresubstrate is removed, such as by laser lift-off.

The steps 112 and 114 may be reversed if the ion implantation occursafter substrate removal, as in FIG. 12B.

If PEC etching is needed, then, in step 116, a bias voltage (e.g.,ground) is applied to the semi-insulating layer via the N-contacts ofthe LED layers during a PEC etch of the exposed layer. Leads on thesubmount wafer may be used for applying the bias voltage to thesemi-insulating layer.

In step 118, the leads on the submount wafer for biasing thesemi-insulating layer are cut or blown.

In step 120, the submount wafer is diced to form LED modules or units,each module containing a group of interconnected LEDs, such ascontaining one or more strings of LEDs for coupling to a relatively highvoltage supply.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications may be made without departing from thisinvention in its broader aspects and, therefore, the appended claims areto encompass within their scope all such changes and modifications asfall within the true spirit and scope of this invention.

The invention claimed is:
 1. A module containing a plurality of lightemitting devices (LEDs), the module comprising: electrically isolatedindividual LEDs, each LED including an N-layer, an active layer, and aP-layer; a semi-insulating isolation layer common to the individual LEDsthat mechanically couples together the individual LEDs and has aresistivity between 10⁴ and 10⁶ Ω-cm; and a metal pattern that connectsthe individual LEDs, at least two of the LEDs being connected togetherin series.
 2. The module of claim 1 wherein the individual LEDs areseparated by trenches formed at least through the P-layer and activelayer.
 3. The module of claim 1 wherein the individual LEDs areseparated by semi-insulating isolation regions formed by ionimplantation.
 4. The module of claim 1 including a submount thatsupports the module of individual LEDs.
 5. The module of claim 4 whereinthe submount includes at least a portion of the metal pattern thatconnects the individual LEDs.
 6. The module of claim 1 wherein the metalpattern connects the individual LEDs in a combination of series andparallel connections.
 7. The module of claim 1 wherein the metal patternconnects the individual LEDs in an anti-parallel configuration, suchthat portions of the individual LEDs are alternatively activated duringalternating positive and negative cycles of a power source.
 8. Themodule of claim 1 wherein a surface of the semi-isolating layer isetched.
 9. The module of claim 1 including a wavelength conversionlayer.
 10. A module containing a plurality of light emitting devices(LEDs), the module comprising: electrically isolated individual LEDs,each LED including an N-layer, an active layer, and a P-layer; asemi-insulating isolation layer common to the individual LEDs thatmechanically couples together the individual LEDs; and a metal patternthat connects the individual LEDs, at least two of the LEDs beingconnected to ether in series wherein the semi-insulating layer includesa semiconductor layer with defects in a crystalline structure of thesemiconductor layer.
 11. A module containing a plurality of lightemitting devices (LEDs), the module comprising: electrically isolatedindividual LEDs, each LED including an N-layer, an active layer, and aP-layer; a semi-insulating isolation layer common to the individual LEDsthat mechanically couples together the individual LEDs; and a metalpattern that connects the individual LEDs, at least two of the LEDsbeing connected together in series, wherein the semi-insulating layerincludes an ion-implanted semiconductor layer.
 12. A wafer comprising: aplurality of modules, each module including a plurality of electricallyisolated individual light emitting devices (LEDs) and a metal patternthat connects the individual LEDs, at least two of the LEDs beingconnected together in series; and a semi-insulating isolation layercommon to the plurality of modules that mechanically couples togetherthe modules and has a resistivity between 10⁴ and 10⁶ Ω-cm.
 13. Thewafer of claim 12 wherein the individual LEDs are epitaxially grown overthe semi-insulating layer.
 14. The wafer of claim 12 wherein thesemi-insulating layer is at least two microns thick.
 15. The wafer ofclaim 12 wherein the semi-insulating layer is sufficiently conductive tosupport a photo-electrochemical etch process while being sufficientlyinsulating to prevent conduction between the individual LEDs.
 16. Thewafer of claim 12 wherein the semi-insulating layer includes asemi-insulating GaN layer.
 17. The wafer of claim 12 wherein a surfaceof the semi-insulating layer is photo-electrochemically etched.
 18. Thewafer of claim 12 including a wavelength conversion layer.
 19. A wafercomprising: a plurality of modules, each module including a plurality ofelectrically isolated individual light emitting devices (LEDs) and ametal pattern that connects the individual LEDs at least two of the LEDsbeing connected together in series; and a semi-insulating isolationlayer common to the plurality of modules that mechanically couplestogether the modules, wherein the semi-insulating layer includes anion-implanted semiconductor layer.